This invention relates to gas analyzing systems and especially such systems wherein it is desired to determine quantitative values of a mixture of gases. Applications of such systems include respiratory function measurements in the medical field wherein a pulmonary function is evaluated such as in the measurement of total lung volume, as distinguished from residual or useful volume, and, in the measurement of oxygen carbon-dioxide respiratory exchange.
The principles upon which the present invention is an improvement are known and is based on the differing sonic velocities of a sound wave in different gaseous mixtures. As is known, the speed of sound in air versus the speed of sound in a mixture of air and another gas component can differ by a substantial amount. For example, if a sonic standing wave pattern is established through a column containing a mixture of helium in air, a corresponding difference in basic wave-length is established and can be measured electronically.
Advantage taken of these differences in the speed of sound through varying mixtures in the present invention to develop a rapidly readable, analog output signal by use of rapid acting electronic signal processing circuits.
In one form of known apparatus described in U.S. Pat. No. 2,984,097 issued May 16, 1961 to Michael Kniazuk a large number of standing waves (200-600) were established in a column in a tube having a transmit transducer coupled at one end and a receive transducer coupled at the other end. The shift in wave length caused by introducing a gaseous mixture different from a reference gas was manually counted by visually reading the number of 360.degree. or 180.degree. phase inversions of a Lissajous figure displayed on an oscilloscope trace. Additional fine reading was obtained by manually adjusting a phase shift capacitor in an associated circuit to bring the Lissajous figure to the nearest nul value after which the unknown amount of the gas introduced into the mixture is determined by adding the manually derived count from the oscilloscope trace to the manually derived shift in phase caused by the manually adjustable capacitor. Thus, the capacitance of phase shift and total number count of scope inversions provided in that system a measure of the unknown amount of gas. Such a procedure is inherently slow, is time-consuming of operator time and is entirely manual and therefore subject to operator error. There is, therefore, a need for a new and improved gas as an analyzing system.